Implement generateblocks rpc call in rpcclient

[arshbot]

In bitcoind 0.21.0, the generateblocks rpc command was introduced. Some documentation can be found here.

While this command isn't useful for mainnet nodes, it can be very useful for local testing, as the generateblocks rpc command accepts an array of transactions which are to be include in the block. To my knowledge this is the only way to control which transactions are mined in a local regtest environment, which could be very beneficial for integration testing.

[cubancorona]

Did, you try,,,, re star-ar-arrr tiiiiing 🎼🎶🎵🎤🎧